On some early maps Duncan Lake is shown with the name "Upper Kootenay Lake" or "Howser Lake", but since 1912 the name Duncan has prevailed.[5]

Before Duncan Dam was built the river served as the main navigation route into the valley, used by mining and logging industries. The route was navigable via Kootenay Lake as far as Bonner's Ferry, Idaho, on the Kootenai River.[6]

Duncan Dam was finished in 1967, the first of the three Canadian Columbia River Treaty dams to be built. Its purpose is to regulate flow into Kootenay Lake, in coordination with Libby Dam, to assure sufficient water flow for the Kootenay Canal and Corra Linn Dam projects. Duncan Dam does not produce any electricity. Duncan Lake, originally 25 kilometres (16 mi) in length, is now 45 kilometres (28 mi) long.[6]